I came across this age-old dilemma recently while working on the design of a major online banking site.  Choosing which personal pronoun to use is deceptively complex and requires far more consideration than it would first seem.

Ultimately I think the most important guideline is to only use a personal pronoun where absolutely necessary to communicate meaning.  Your first question should be “Do I need a personal pronoun?”  And only then “which one should it be?”  It’s a slippery slope when you start adding an empty modifier to all of your labels for consistency and end up murdering scanability.  You need to ensure its adding value.

The usual sage advice people like to prescribe a problem like this is “it doesn’t matter which one you choose, just be consistent”.  This advice is well meaning but entirely useless.  Unless your site does almost nothing you’ll inevitably end up with cases where you have to use “your” regardless of which one you choose.  This is because the scope of “my” is fundamentally limited to the few objects or items that can be considered an extension of the user.  The interaction will become a dialog eventually, at which point “my” breaks down and just starts to sound ridiculous.  So if you go with “My” don’t be deceived into thinking that its simply a find and replace.  Each instance needs to be considered in its own right.

As for whether you should start with “my” or “your”, consider what the metaphor for the interaction is (I’m talking about a very basic metaphor for the interaction here, not some kind of overarching navigational or behavioural metaphor).  If you’re using an iPhone it is one of operating a machine so “my” applies broadly and works well (but still only appearing where absolutely necessary).  The “My number” at the top of the contacts screen is a good example of this – only the use of a personal pronoun could communicate what that number is.  You don’t think of it as a dialog between you and Apple.  Its the same kind of interaction that you’d have with your car – you drive your car, you don’t have a conversation with it.

On the web however things are not so clear cut.  You end up with spaces that are a bit of both.  Think about online banking.  Is it an intimate, private space or is the metaphor one of interacting with a teller/assistant?  I think the answer is both.  When it comes to selecting a personal pronoun, you need to consider both the brand/site *and* the nature of the specific interaction.

There is also a brief treatment of this topic on the Yahoo Design Pattern Library.